(1) Field of the Invention
The present invention relates to a plastic gate valve mainly used in a large-diameter passage of liquid transport piping for liquid such as slurry.
(2) Description of the Related Art
A gate valve is known which opens and shuts a passage by moving a cylindrical or plate-shaped valve body in a valve casing for control of a flow of liquid. The gate valve is generally made of metal or plastic. When the gate valve is used in liquid transport piping for liquid such as slurry, the valve body of the gate valve is covered with an elastic material such as rubber.
If the gate valve is made of metal, the valve body is generally covered with a rubber-like elastic material by vulcanization adhesion. This is satisfactory in itself, but there are other problems with metal valves, such as rust mixing into the liquid and greater heaviness than plastic.
If the gate valve is made of plastic, such as rigid polyvinyl chloride, there is no problem with rust or heaviness, but the plastic cannot be heated to the temperature for vulcanization adhesion so it is difficult to cover the valve body with a rubber-like elastic material so as to be integral with the valve body. This means the rubber-like elastic material must be adhered to the plastic valve body by an adhesive. Thus, method of affixment requires special, expensive adhesives and various treatment of the valve body to ensure sufficient interfacial adhesive strength. Without the same, the rubber-like elastic material often peels off due to the liquid pressure, clogging the passage. Also, when the gate valve is opened, biased expansion of the rubber-like material occurs due to the liquid pressure. Further, peeling of the rubber-like elastic material from the valve body occurs due to the frictional resistance between the valve body and the valve seat of the valve casing. As a result, the sealing effect in the gate valve is decreased.
Some improved plastic gate valves are available in which the plastic valve body is covered with metal by adhesion and the metal is covered with the rubber-like elastic material by vulcanization adhesion. However, in this type of valve body construction, the adhesion between the metal and plastic is insufficient. If a large impact is applied to the materials, they easily separate. Also, rust makes it impossible to use the above gate valve for a long time as the rust will mix into the liquid and the metal will corrode to an usable state.